Method for assembling conductive particles into conductive pathways and sensors thus formed

ABSTRACT

A sensor is achieved by applying a layer of a mixture that contains polymer and conductive particles over a substrate or first surface, when the mixture has a first viscosity that allows the conductive particles to rearrange within the material. An electric field is applied over the layer, so that a number of the conductive particles are assembled into one or more chain-like conductive pathways with the field and thereafter the viscosity of the layer is changed to a second, higher viscosity, in order to mechanically stabilise the material. The conductivity of the pathway is highly sensitive to the deformations and it can therefore act as deformation sensor. The pathways can be transparent and is thus suited for conductive and resistive touch screens. Other sensors such as strain gauge and vapour sensor can also be achieved.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 13/992,517, filed on Jun. 7, 2013, which is a 35 U.S.C. § 371national stage patent application of international patent applicationPCT/EP2011/072113, filed on Dec. 7, 2011, which claims priority toSweden patent application 1151005-4, filed Oct. 27, 2011, and Norwaypatent application 20101714, filed on Dec. 8, 2010.

TECHNICAL FIELD

The invention concerns a method for forming a well-defined micrometerscale conductive wire or pathway comprising conductive particles on ageneric substrate using an electric field. The pathway can be covered bynon-conductive organic material like polymer. The pathway connects itsending points electrically, but the strength of this connection can varywith the substrate deformations, the pathway thus acting as adeformation sensor as the conductance and admittance change. The pathwaystructure is more sensitive to the deformations compared to the randomlyoriented mesh. Pathways may also act as a capacitive sensor, and thefeatures can be combined in a hybrid sensor. Such sensors can be used indifferent areas including nanomechanical cantilever sensors, cantileversensors and touchscreens.

BACKGROUND OF THE INVENTION

Electrically conductive organic material containing materials can bebased on the mixture of polymer-containing matrix and conductiveparticles (fillers) embedded into this matrix. In the former case thematrix can also contain organic or inorganic additives and theelectrically conductive particles either carbon, metal or metal oxideparticles. The materials can also be directionally conductive.

In Sensors 2008 8 1595 (Maria Nordstrom & al. published 10 Mar. 2008ISSN 1424-8220) it is illustrated how the conductivity of organicconductive layer on the substrate can vary with the deformations of thesubstrate. If the substrate deformations vary as a response to anexternal input, the conductivity variation can be used to sense thisinput, and the whole system forms a sensor. Ultimately, the layer can belocated on a force microscope cantilever, particularly on an atomicforce microscope (AFM) cantilever, whose tip is detecting a samplesurface which acts as an external input for cantilever deformation.

In these cases, the conductive organic layer contains conductiveparticles or polymers that form a random network. When the substrate isbending in one direction, the conductive paths are loosened or broken inthis direction but not perpendicular to it. Thus, the conductivity ofthe organic layer is not anisotropic in the first place, and theparticle connections perpendicular to the deformation are essentiallynot influenced by deformation and are thus not contributing to thesensing phenomenon. There is a need for a sensor where the pathways aredirected in this same direction, parallel to the deformation, thus beingmore sensitive to substrate bending.

A touch screen is an electronic visual display that can detect thepresence and location of a touch within the display area using e.g. afinger, a hand or a stylus. Touch screens are used in many digitalappliances, such as mobile phones, personal computers, electronic books,satellite navigation devices and video games.

A resistive touch screen panel is usually composed of layers using twothin, electrically conductive layers separated by a narrow gap. When anobject, such as a finger, presses down on a point on the panel's outersurface, the two layers become connected at that point: the panel thenbehaves as a pair of voltage dividers with connected outputs. Thiscauses a change in the electrical current, which is registered as atouch event and sent to a controller for processing. A resistive touchscreen can also be piezoresistive; when pressed the conductivity of thematerial or wires increase and the controller detects where.

A capacitive touch screen panel consists of an insulator such as glass,coated with a transparent conductor such as indium tin oxide. As thehuman body is a conductor, touching the surface of the screen results ina distortion of the screen's electrostatic field, measurable as a changein capacitance. Different technologies known to someone skilled in theart may be used to determine the location of the touch. The location isthen sent to a controller for processing. Examples of controllers are 3MTouch EX II 7000 Series for a capacitive screen or Semtech SX8650 for aresistive screen.

A touch screen can also combine capacitive and resistive sensing, e.g.detecting the proximity of one or more fingers, resulting in one action,and detecting one or more fingers tapping the screen resulting inanother action. US20090189875A1 teaches one method for constructing ahybrid touch screen.

Problems with the current technologies for producing touchscreen panelsare that transparent conductors such as indium tin oxide are expensiveand have limited durability, and that resistive screens must be producedwith several layers.

Nanomechanical cantilevers can be produced as micro fabricated siliconbeams or using piezoelectric materials, as taught in U.S. Pat. No.7,458,265. They are used as mechanical sensors, useful as mass andviscosity sensors, which transform processes occurring at their surfaceinto a mechanical response. This signal transduction principle allowssurface stress measuring at the cantilever surface by monitoring thebending of the cantilever and at the same time observing changes in theoscillation properties of the cantilever related to changes in mass loadon the cantilever. Nanomechanical cantilevers can be used for chemicalsensing such as detection of heavy metals, and as biosensors, e.g., forDNA and protein detection. Arrays of cantilever sensors can be employedfor the parallel detection of multiple molecules of interest. Also,nanomechanical cantilever sensors can be used in surface and materialssciences for the real-time monitoring of self-assembled monolayer (SAM)formation, the detection of cholesterol interaction with hydrophobicsurface layers and to study layer-by-layer build-up processes inreal-time are possible, refer to Journal of Nanoscience andNanotechnology, Volume 10, Number 4, April 2010 Koeser, Joachim & al:Nanomechanical Cantilever Sensors as a Novel Tool for Real-TimeMonitoring and Characterization of Surface Layer Formation.

Touch sensors can be made by using Quantum Tunneling Composites (QTC)manufactured by Peratech Ltd. QTC is made from conductive fillerparticles, e.g. metal particles, combined with an elastomeric binder,typically silicone rubber, disclosed in WO/1999/038173. The metalparticles are given an irregular structure with a spiked surface whichis electrically insulated by the silicone rubber. The rubber allows theparticles to get close but not touch even when the material is pressedor densely loaded.

Increased charge on the spikes decrease the effective width of thepotential barrier in quantum tunneling. This reduces the distance andenergy required for the electron charge to tunnel through, and thematerial becomes conductive. The system with spiky particles is far moresensitive than the system with rounded particles would be. Theconductance varies with the dynamic conditions. QTC are in generalisotropic. There is a need for making anisotropic QTC in order tomanufacture improved sensors.

“Multifunctional Chemical Vapor Sensors of Aligned Carbon Nanotube andPolymer Composites” by Wei & al. J. Am. Chem. Soc., 2006, 128 (5), pp1412-1413, DOI: 10.1021/ja0570335 describes how partially coatedperpendicularly aligned carbon nanotube arrays with an appropriatepolymer thin film along their tube length can be used as sensors forchemical vapours, and also for mechanical deformations, thermal andoptical exposures.

U.S. Pat. No. 7,777,478 discloses sensors based on nanotubes that can beused as touch and auditory sensors. In a similar way, U.S. Pat. No.7,673,521 teaches how carbon nanotubes are grown from organometallic andincorporated into a polymer matrix to form a nanosensor which providesinformation regarding a physical condition of a material such as anairplane chassis or wing, in contact with the nanosensor.

There is a need for a better way to grow aligned conductive paths in amatrix, to form such sensors.

The object of the invention is to fulfil one or more or theabove-mentioned needs, or to provide a useful alternative to existingmethods and products.

DESCRIPTION OF THE INVENTION

The invention provides for a method for forming a sensor on a substratein accordance with claim 1. In accordance with the method, one or moreconductive wires or pathways of conductive particles are formed.

The conductive pathway can be embedded in a matrix. The matrix could beorganic, like a polymer, or inorganic, like inorganic polymer or glass.The pathways are formed located on a substrate covered with matrix, in amatrix that is or becomes a part of the substrate, or placed on asubstrate after being formed in a matrix.

The term “substrate” is in this disclosure used for all theseembodiments, and the term “matrix” may include the substrate.

The terms “wire” and “pathway” and “string” are used interchangeably.Moreover, the terms “single wire” or “a stringlike formation” are usedinterchangeably.

The term “conductivity” is used for both electric conductance andelectric admittance. The inventions can thus be used for both sensingchanges in direct (DC) or alternate current (AC).

The term “sensor” is used both for the pathway itself, as itselectromagnetic properties are changed, as disclosed below, and for thecomplete arrangement that includes the controllers and auxiliarycircuits that is needed to form a sensor that can be applied e.g. as atouch sensor or a cantilever sensor.

The conductivity of the pathways is varied with deformation of thesubstrate. The substrate can be deformed by pressing. The substrate canalso be deformed from bending or pulling or by other forces, such asgravity or electromagnetic forces. The conductivity change is in thealignment direction of the one or more pathways, which makes thepathways act as a deformation sensor.

The pathways can also be used as a capacitive sensor using capacitivecoupling between the pathway and an object or human. Capacitive sensorsdetect anything which is conductive or having dielectric properties.Multi-touch and gesture-based touchscreens are premised on capacitivesensing and can be made using the present invention.

A combined resistive and capacitive sensor can also be produced, usefulfor touchscreens.

Conductive particles in a matrix can be assembled by an electric fieldinto aligned pathways. After the alignment step the matrix and thusconductive pathways is stabilized for example by curing (thermosetmatrix), by lowering the temperature (thermoplastic or thermotropicmatrix or glass) or by evaporating the accompanying solvent off(lyotropic matrix). The pathways can remain embedded in the substrate orbe exposed on its surface, if parts of the matrix is removed or cleaved,e.g. by etching or burning or pyrolysis.

The method comprises the steps of first applying a layer of a matrix andconductive particles over a substrate, when the viscosity of the matrixis low enough to allow the conductive particles to rearrange within thematrix. In a subsequent step an electric field is applied over thematrix, resulting in a number of the conductive particles aligning withthe field and thus forming conductive pathways. Thereafter the viscosityof the matrix is changed to a second, higher viscosity, in order tomechanically stabilise the pathways.

Preferably, the conductive particles are formed by infusible particlessuch as carbon particles or metal oxide or metal particles that havedielectric properties such that they are aligned by a field.Advantageously, the conductive particles show low molecular or particleanisotropy, and thus the major part of the conductive particles has alow aspect ratio; aspect ratio ranges of 1-4, or 1-5, 1-10 or 1-20 aretypical. The terms “low molecular or particle anisotropy” and “lowaspect ratio” has the same meaning herein. This is the case for examplewith irregular graphitic particles, spherical carbon black (CB) ordisk-like or conical carbon particles here referred to as carbonnanocones (CNC).

The conductive particles can be a mixture of different carbon particles.Also other conductive particles can be used, like metal, such as silveror metal oxide particles or colloidal metal particles. Dielectricparticles that are piezoelectric, like lead zirconate titanate (PZT),barium titanate, strontium titanate, lead magnesium niobate, leadtitanate solid solutions, strontium lead titanate and lanthanum galliumsilicate could be used for giving the pathways piezoelectric features.

The matrix can be a thermoset polymer, which means that it is stabilizedby curing which forms permanent chemical cross-links. It can be athermoplastic polymer system which means that its viscosity is lower athigher temperatures, which allows alignment of particles, and higher atlower temperatures, which allows stabilization of the matrix afteralignment. The matrix can be a lyotropic system which means that thematrix can be plasticised by solvent and solidified by evaporating thissolvent off.

It can also be any combination of these systems. For example, thelyotropic matrix can contain thermoset polymer that can contain solventfor plasticizing it, but the stabilization can be based both oncross-linking the polymer and also on the solvent evaporation.

The matrix can also comprise a UV-curable polymer, in which case theviscosity may be altered by submitting the matrix to light in the UltraViolet range. UV-curing is sometimes referred to as photoinitiatedpolymerization. A UV-curable matrix material may be combined with any ofthe above-mentioned types of matrix materials to form variousalternative matrix materials.

Moreover, the matrix may comprise an elastomer. This provides forpossibilities of creating a compressible matrix where the resistancethrough the conductive path may be decreasing with the compression ofthe matrix.

The electric field can be created between electrodes that are placed indirect contact with one or both sides of the substrate and matrix,constituting a first layer. The electrodes can also be placed outsideadditional insulating layers, where the insulating layers are placed incontact with the first layer. The purpose of the insulating layers canbe to build capacitive sensors or to build structures where the pathwaysare partially exposed to other conductive wires or layers and partiallyinsulated from them. The electrodes can also be remote, not in directcontact with the layers.

The direction of the electric field is determined by the electrodearrangement, and thereby the direction of the conductive pathways formedby the aligned conductive particles can be controlled. The pathways canbe formed in any direction in the matrix. Pathways can also be formed bymoving the electrodes, thus forming pathways that can act as coils orantennas. The pathways can be controlled in three dimensions within thematrix by moving the electrodes and changing the field, or multiplelayers can be used.

The electric field can be in the order of 0.05 to 35 kV/cm, or morespecifically 0.1 to 10 kV/cm. This means that for a typical alignmentdistance in the range of 10 μm to 1 mm, the voltage applied can be inthe range of 0.1 to 100 V. The field may be an alternating (AC) field,but can also be a direct (DC) electric field. A typical field is an ACfield having a frequency of 10 Hz to 10 MHz. Very low frequencies <10 Hzor DC fields lead to asymmetric chain formation and build up. The lowvoltage needed for applying the method is simple to handle in aproduction line and does not need the specific arrangements necessarywhen handling high voltages.

Thus, the present invention is based on the finding that it possible toalign conductive particles in fluid-like matrices using an electricfield to form conductive pathway in the fluid-like polymer matrices. Thepathway is able to enhance the macroscopic conductivity of the material.In particular, the formation of conductive pathways allows the materialto become conductive also when it contains a lower amount of conductiveparticles than what is otherwise necessary for creating electricalcontact for the material having randomly distributed particles.

This procedure renders an anisotropic material and a directionalconductivity that is higher along the alignment direction thanperpendicular to it.

The method can be used to produce a variety of sensors. Possibleapplications of sensors according to the present invention include butare not limited to:

-   -   Touch screens    -   Cantilever sensor for nanomechanical detection    -   Structures in microelectromechanical systems (MEMS).    -   Cantilever arrays as biosensors for medical diagnostic        applications    -   Cantilevers as radio frequency filters and resonators    -   Cantilever transducers for atomic force microscopy    -   Moisture detectors    -   Strain gauges    -   Nonintrusive bio monitoring

A sensor can be formed by connecting a pathway connecting to two orseveral electrodes on the substrate thus providing an electricalconnection between the electrodes. When the substrate is deformed by anexternal input in the alignment direction of the pathway, this increasesdistance between the particles at one or more points of the pathway andthe conductivity is sharply decreased. Thus, it is possible to measuresubstrate deformation, such as bending, and thus the external inputcausing the bending. Depending on the geometry of the pathway and howthe deformation is applied, there could also be a decrease of thedistance between the particles and conductivity could increase, so thatthe pathways are not conductive until the matrix is deformed. All thesechanges are detected by the controller and analysed by the controller ora connected processor, embedded system or computer.

In the case of the pathways being used as a capacitive sensor, it is thecapacitance, i.e. the charge held by the pathways, which is changed byconductive coupling from another object or a human. This change is thendetected by connected controllers such as 3M Touch EX II 7000 Series,Freescale Semiconductor MPR121 Proximity Capacitive Touch Sensor. Thecontroller is generally a microcontroller-based integrated circuitplaced between the sensor (the conductive pathways with the substrateand the circuits that connect them) and a processor such as an embeddedsystem controller or a computer. The controller reads electricinformation from the sensor and translates it into information suitedfor the PC or embedded system controller. This information can e.g. bethe coordinates for where a finger or object is near or touches thesensor, the strength of the touch, or the number of touches and how thetouch moves, as needed for multi-touch applications.

For resistive sensor applications the controller is similar, but itdetects changes in conductivity. Some controllers can be strapped to actas either a resistive or capacitive controller, e.g. the 3M Excaliburand ExII chipset. These or other controllers can be used to make ahybrid resistive and capacitive sensor, e.g. for use as a touch screen.

Cantilevered beams are commonly used in the field ofmicroelectromechanical systems (MEMS). MEMS cantilevers can befabricated from polymers. They can include conductive pathways of thepresent invention. The fabrication process involves undercutting thecantilever structure to release it, for example with a wet or dryetching technique. When the cantilever vibrates, the vibrations changesthe conductivity and capacitance of the pathway, and this can bedetected by a controller.

Humidity sensors are generally manufactured using capacitors andresistors. If the matrix consists of a polymer material that ispermeable to vapour, e.g water vapour or vapours of alcohols, e.g. usinga biopolymer such as cellulose, the conductance and capacitance of theconductive pathways will change when a voltage is applied, and this canbe detected by a controller. A strain is a normalized measure ofdeformation representing the displacement between particles in the bodyrelative to a reference length. A strain is in general a tensorquantity. A strain gauge is a device used to measure the strain of anobject. It can be made using the present invention where the conductivepathways change their conductivity as the object to which the substrateis fastened changes in size and deforms the substrate of the straingauge. The change of conductivity can be measured using a Wheatstonebridge. The pathways can be straight or have a pattern.

Sensors of the present invention can be used for nonintrusive biomonitoring. For example a pulse can be measured as strain on the skin.Heartbeat and blood flow will result in change of capacitance that canbe measured with the present invention configured as a capacitivesensor.

In a particular embodiment, it is suggested to form a conductive pathwaycomprising CB (carbon black) particles, most preferred assembled into asingle wire, in a polymer matrix. Preferably, the polymer may be an UVcurable thermoset polymer, and advantageously with a glass transitionpoint below room temperature.

Advantageously, a micro-mechanical strain sensor may be produced made ofCB particles, most preferred assembled into a single wire, in a polymermatrix.

Aligned single strings of CB in polymer as well as polymer compositescontaining CB strings can mimic the piezoresistive properties of carbonnanotubes (CNTs) and the polymer composites containing CNTs, but CBparticles provide additional benefits including significantly easiermixing and lower production costs.

Aligned strings make an initially insulating composite conductive andthe stretching of strings result in a piezoresistive effect due to theinduced displacement of the articles. The strings show gauge factors ofabout 150 while corresponding films containing 12 vol. % of CB arealmost completely insensitive to identical stretching. A gauge factor of150 significantly exceeds the values of 15-20 previously shown in theprior art for isotropic CB in SU8 polymer (see L. Gammelgaard et al.Appl. Phys. Lett. 88 (2006), 113508).

It is believed that the gauge factor may be varied by varying theparticle sizes, particles size distribution and/or by improving theconductivity of the particles.

Advantageously, there may be provided a conductive pathway comprising CBparticles in a matrix comprising an elastomer.

In another embodiment, it is proposed to form a conductive pathwaycomprising carbon nanocones and discs (CNCs), being aligned into astringlike formation, preferably using an alternative electric field(dielectrophoresis).

Carbon nanocones and discs (CNCs) have intriguing properties includingan unique conical topology. CNCs are formed by stacked graphene cellswith a tip of 1-5 carbon pentagons, which allow only discrete apexangles for the cone opening: 112.9°, 83.6°, 60.0°, 38.9° and 19.2°.

Advantageously, the CNC particles may initially be dispersed into apolymer matrix with a particle fraction below the percolation thresholdof the CNCs. In a particular embodiment, the percolation threshold isabout 2 vol. %, and it is suggested to use CNC particles dispersed toless than about 1 vol %, less than about 0.1 vol. % or even less than0.01 vol %.

A value well below the percolation threshold will suppress particleaggregation and facilitate transparency of the matrix, which isadvantageous as it allows the use of an UV-curable polymer for thematrix.

Advantageously, the alignment may be performed using a AC or DC electricfield, most preferred an AC field. The voltage may preferably be asmentioned above, 0.05-35 kV/cm, more preferred 0.1 to 10 kV/cm. For analternating field the frequency may advantageously be as mentionedabove: 10 Hz to 10 MHz: In a preferred embodiment using CNC particles 1kHz and 4 kV/cm was used. The alignment of the particles may develop inminutes and makes the initially insulating, nonaligned materialconductive.

The following curing, preferably UV-curing of the polymer matrix,renders a solid state device.

The stretching of the aligned strings in the cured polymer leads to areversible piezoresistive effect, and a gauge factor of about 50 hasbeen demonstrated. This is in sharp contrast to prior art CNC films witha particle fraction above the percolation threshold (13 vol. %), whichare conductive but not sensitive to stretching.

The resulting CNC strings are Ohmic in nature and may show higher DCconductivity (example 22-500 S/M) than identically prepared strings outof carbon black particles (CB) (example 1-22 S/m) (see examples below).

In particular it is suggested to form a micro-mechanical strain sensorbased on carbon nanocones and discs (CNCs) which are aligned intostringlike formation as described above.

In addition, it may be mentioned that the methods and sensors describedin the above are believed to be suitable for different purposes,including strain, stress or force sensors, for example such as describedin WO 2011/079390. In particular, the method may comprise the formationof three dimensional networks of pathways, resulting in sensorscomprising such three-dimensional pathways. This type of structures maybe particularly advantageous for use in robot skin or machine or devicesurface applications.

Printing techniques can be used to manufacture layered objects,including mechanical and electronic systems. Thus, using embodiments ofthe present invention, it is possible to create aligned strings in sucha printed layer matrix, or to modify printed strings, e.g. by healing apoorly printed string, or by adding particles of different material,where some could be printed, and some could be in the printed matrix.U.S. Pat. No. 7,766,641B2 “Three dimensional (3D) printer system withplacement and curing mechanisms” discloses one example of a printersystem with a curing mechanism, that could be used for this purpose.

Moreover, the methods and sensors described herein are believed to besuitable in shear sensors, such as for example those described in U.S.Pat. No. 5,313,840. Here a tactile sensor capable of detecting shearforce comprises an anisotropically conductive material disposed betweena conductive cursor and an array of contacts. In one embodiment, theanisotropic material is affixed to the contact array, and the cursor isaffixed to an elastomeric skin overlying the material. Movement of thecursor is detected b interconnection of the contacts underlying thecursor. In another embodiment, the anisotropic material is affixed tothe cursor but is free to move over the contact array in response toshear force.

Movement of the cursor is detected by interconnection of the underlyingcontacts. Such arrangements can also detect pressure and temperature.

In addition, sensors in accordance with the sensors described hereincould be used in several other applications, such as for example:

-   -   Sensors integrated in rubber shoe soles or gloves for monitoring        stress,    -   Sensors integrated in clothing for monitoring touch, movement or        tear.    -   Sensors integrated in tires such as car/aircraft tires, using        homogenous stress monitoring and/or detection of weakness and of        wear and tear,    -   Stress and shear sensors for large structures such as buildings,        ships, aircrafts or cars.    -   Shear measurement sensors used for patients in healthcare    -   Sensors for wall shear stress and other shear stress        measurements related to engines and machines, e.g along the        combustor wall of an aircraft.

Hence, in a first aspect of the invention, there is provided method forforming a sensor on a substrate, using electrodes forming one or moreanisotropic conductive pathways in one or more layers, from mixturecomprising matrix and conductive particles comprising the steps

-   -   a) forming a layer of the mixture, the mixture having a first        viscosity which allows the conductive particles to rearrange        within the layer;    -   b) applying an electric field over the layer, so that a number        of the conductive particles are assembled and aligned with the        field, thus creating one or more conductive pathways;    -   c) changing the viscosity of the layer to a second viscosity,        said second viscosity being higher than the first viscosity, in        order to mechanically stabilise the layer and preserve the one        or more conductive pathways.

In certain embodiments, the matrix may be totally or partly removed fromthe layer after step c.

Advantageously, the conductivity of the pathways is changed if thematrix is deformed.

Preferably, the particles comprise material selected from the groupcarbon, metal, metal oxides, ceramics, piezoelectric material.

Advantageously, the particles are conductive from quantum tunnelingeffects.

Preferably, the number of particles in step a) is below a percolationthreshold. It is particular advantage of the method proposed herein thatthe concentration of conductive particles may be low. For conductivemixtures, a percolation threshold is defined as the lowest concentrationof conductive particles necessary to achieve long-range conductivity ina random system. The percolation threshold may thus be determinedexperimentally for a particular combination of matrix and conductiveparticles.

In a system formed by a method as proposed herein, the concentration ofconductive particles necessary for achieving conductivity in apredefined direction may be lower than the percolation threshold, whilestill providing the necessary conductivity.

For practical reasons, the concentration of particles is determined bythe requirements on the conductive pathways. There is usually no reasonto have excess amounts of conductive particles not arranged into theconductive pathways. The concentration of conductive particles in thematrix could be up to 10 times lower than the percolation threshold.Concentrations of conductive particles may for example be in the rangeof 0.2-10 vol %, or 0.2-2 vol % or 0.2-1.5 vol %.

The electric field may advantageously be generated between one or morepairs of alignment electrodes that are at fixed position or that aremoved relatively to the substrate.

At least one of the alignment electrodes may be in direct contact withthe layer.

Alternatively, the alignment electrodes may be insulated from the layer.

The method may comprise the step of applying either AC or DC-electricfield in the order of 0.05-35 kV/cm, and especially in the order of0.1-10 kV/cm.

Advantageously, the particles have an aspect ratio range of 1-20, morepreferred 1-10, more preferred 1-5, most preferred 1-4.

Advantageously, the particles comprise irregular graphitic particles,spherical carbon black (CB) particles or disc-like or conical carbonparticles (carbon nanocones CNCs).

Preferably, the matrix is a thermoset polymer, a thermoplastic polymersystem, a lyotropic system or a mixture thereof.

Alternatively or in addition thereto, the matrix may comprise aUV-curable polymer.

Alternatively or in addition thereto, the matrix may comprise comprisesan elastomer.

The above-mentioned features of a method may be combined to form variousembodiments of the invention.

In another aspect of the invention there is provided a sensor beingmanufacturable by the method in accordance with the above.

In another aspect of the invention, there is provided a sensormanufactured by the method in accordance with the above.

In another aspect of the invention, there is provided a sensor with oneor more conductive pathways formed from conductive particles in a matrixwherein the number of particles in the pathways is below the number ofparticles that constitutes a percolation threshold if the particles werehomogenously distributed in the matrix.

Advantageously, the sensor comprises a substrate, and the conductivityof the aligned particles is influenced by the substrate deformations orbending.

Advantageously, the capacitance of the pathways may be influenced by thepresence of an object or a human.

Preferably, the particles have an aspect ratio range of 1-20, morepreferred 1-10, more preferred 1-5, most preferred 1-4.

Advantageously, the particles comprise irregular graphitic particles,spherical carbon black (CB) particles, or disc-like or conical carbonparticles (carbon nanocones CNCs).

The matrix may advantageously be a thermoset polymer, a thermoplasticpolymer system, a lyotropic system or a mixture thereof.

Alternatively or in addition thereto, the matrix may comprise aUV-curable polymer.

Alternatively or in addition thereto, the matrix may comprise anelastomer.

Advantageously, the particles may comprise CB particles, preferablybeing assembled to form a single pathway.

Advantageously, the particles may comprise carbon nanocones and discs(CNCs), preferably being assembled to form a single pathway.

Advantageously, the matrix may comprise a polymer material, preferably aUV-curable polymer material, and most preferred it may also have a glasstransition point below room temperature.

Preferably, the sensor may be a micro-mechanical strain sensor.

Advantageously, the substrate may be an AFM cantilever.

The features of a sensor as described above may be combined with eachother to form advantageous embodiments.

In another aspect of the invention there is provided the use of a sensorn accordance with the above in a resistive, capacitive or hybridtouchscreen.

In another aspect of the invention there is provided the use of a sensorin accordance with the above in a cantilever sensor

In another aspect of the invention there is provided the use of a sensorin accordance with the above in a nanomechanical cantilever sensor

In another aspect of the invention there is provided the use of a sensorin accordance with the above as a vapour sensor.

In another aspect of the invention there is provided the use of a sensorin accordance with the above, where the sensor also functions as atleast one of an anti-static coating, a thermal conductor, an antenna andan electro-magnetic shielding.

In another aspect of the invention there is provided the use of a sensorin accordance with the above in a robot skin application.

The invention will now be further described with reference to exemplaryembodiments and to the drawings, which refer to non-limiting examplesonly, and wherein:

LIST OF DRAWINGS

FIG. 1 shows the method of forming aligned particle wires in between twoelectrodes. The arrow in the Figure indicates the direction of theprocess.

FIG. 2A FIG. shows alignment with electrical contacts betweenelectrodes.

FIG. 2B shows alignment with electrical contacts between electrodes.

FIG. 2C shows alignment without electrical contacts between electrodes.

FIG. 2D shows alignment without electrical contacts between electrodes.

FIG. 3 shows an AFM image of a single aligned particle wire on thegeneric surface.

FIG. 4 shows schematics of a single wire on an AFM cantilever.

FIG. 5 shows schematics of a hybrid touch screen with controllersconnected to a processor of a PC, mobile phone or the similar.

FIG. 6A shows schematics of a strain gauge having a strain sensitivepattern (S) between two terminals (T).

FIG. 6B shows schematics of a strain gauge when the matrix is tensioned.

FIG. 6C shows schematics of a strain gauge when the matrix iscompressed.

FIG. 7 shows dendritic pathways useful for capacitive sensing.

FIG. 8A illustrates schematically an experimental procedure for forminga strain sensor, where a low particle fraction mixture was spreadbetween the electrodes.

FIG. 8B illustrates schematically an experimental procedure for forminga strain sensor, where the low particle fraction mixture was aligned byan E-field between the electrode tips.

FIG. 8C illustrates schematically an experimental procedure for forminga strain sensor, where substrate was bent and the resistance of thestrings was measured as a function of deflection.

FIG. 8D illustrates schematically an experimental procedure for forminga strain sensor, where W(x) is the vertical deflection and L is thelength of the beam clamped at both ends.

FIG. 9A illustrate shows the impedivity of an aligned CB string in curedpolymer.

FIG. 9B shows the phase angle of an aligned CB string in cured polymer.

FIG. 10A illustrates the resistivity of an aligned string of CBparticles as a function of deflection.

FIG. 10B illustrates the relative change in resistance for the alignedstring as a function of deflection.

FIG. 11 illustrates the relationship between resistance and compressionof a sensor in accordance with another embodiment of the invention.

FIG. 12 is an image of multiple aligned strings of CNCs in an embodimentof the invention.

FIG. 13A is a micrograph of the assembly of a string of CNCs before theelectrical field was applied.

FIG. 13B is a micrograph of the assembly of a string of CNCs at 45seconds after 45 seconds.

FIG. 13C is a micrograph of the assembly of a string of CNCs after 1minute and 20 seconds.

FIG. 13D is a micrograph of the assembly of a string of CNCs after 2minutes and 20 seconds.

FIG. 14 illustrates a string of aligned CNC particles in an embodimentof the invention.

FIG. 15 is a comparative figure schematically showing sample fractionversus conductivity for aligned CB and aligned CNC;

FIG. 16 is a comparative figure showing current to voltage or an alignedsample and a non-aligned sample;

FIG. 17 shows the direct current through the sample grows linearly withincreasing voltage up to 100 mV and no sign of hysteresis is observed oncycling.

FIG. 18A shows AC impedivity of aligned CNC film and isotropic CNC film.

FIG. 18B shows the phase angle of aligned CNC film and isotropic CNCfilm.

FIG. 19A illustrates resistivity as a function of deflection for analigned CNC string versus a nonaligned sample.

FIG. 19B illustrates the relative change in resistance as a function ofdeflection for the aligned string.

DETAILED DESCRIPTION OF THE INVENTION

In all embodiments, the method comprises the mixing of infusibleconductive particles and fluid matrix. The matrix contains at leastpolymer and potentially solvent. The electric field aligns theconductive particles mixed in this fluid. Control of the viscosity ofthis mixture is by curing the polymer matrix, e.g. by lowering itstemperature or by evaporating solvent off.

The resultant aligned material retains anisotropic properties and hasdirectional electrical conductivity. In this way, aligned conductivemicrostructures are formed of originally infusible particles.

When the substrate of the aligned pathway is deformed, the conductivitychanges. When a conductive or dielectric body is close to the pathway,the capacitance changes.

The sensor is manufactured by performing the steps of

-   -   a. forming a layer of the mixture, the mixture having a first        viscosity which allows the conductive particles to rearrange        within the layer;    -   b. applying an electric field over the layer, so that a number        of the conductive particles are assembled and aligned with the        field, thus creating one or more conductive pathways;    -   c. changing the viscosity of the layer to a second viscosity,        said second viscosity being higher than the first viscosity in        order to mechanically stabilise the layer and preserve the one        or more conductive pathways.

The matrix may be totally or partly removed from the layer after step c.The steps may be repeated to create several layers. The conductivepathways in one layer can be connected to the pathways in other layers.The field in step b) can be changed and moved.

In another embodiment the matrix is partly removed by using a solvent orheat. The conductive pathways are exposed. The matrix is replaced with apolymer having mechanical properties more preferable for its use as asensor.

The resulting sensor device with one or more conductive pathways formedfrom conductive particles in a matrix can have a number of particles inthe pathways being below the number of particles that constitutes apercolation threshold if the particles were homogenously distributed inthe matrix.

The invention will be further described by the following examples. Theseare intended to embody the invention but not to limit its scope.

Example 1

This example concerns the applicability of the alignment method, the useof alignment for formation of individual aligned chains in thepredetermined positions.

The employed conductive particles were carbon black (CB) from AlfaAesar, carbon nano cones CNC from n-Tec AS (Norway) and iron oxide(FeO·Fe2O3) from Sigma-Aldrich.

The employed polymer matrix was a two component low viscosity adhesiveformed by combining Araldite AY 105-1 (Huntsman Advanced Materials GmbH)with low viscosity epoxy resin with Ren HY 5160 (Vantico AG).

The conductive particles were mixed in the adhesive by stirring for 30minutes. Due to the high viscosity of mixture, efficient mixing ispossible only up to 20 vol-%. of particles.

Estimated percolation threshold of these materials are at ˜2 vol-%. Themixtures are conductive above and insulators below this threshold.Particle loads of 1/10 of the estimated percolation threshold were used.

The particles in the matrix were aligned using an AC source. In thisexample the alignment procedure 1 kHz AC-field (0.6-4 kV/cm, rms value)was employed for >10 minutes for >1 mm electrode spacing and <10 minutesfor <1 mm electrode spacing.

The curing was performed immediately afterwards at 373 K for 6 minutes.

The electrode area is kept sufficiently small to allow only a singlepathway of particles. Alternatively, the particle fraction is lowered.This is shown in FIG. 1.

In one embodiment of this example, metal particles, silver flakes(Sigma-Aldrich) of size 10 μm, was used instead of carbon particles.

Example 2

In FIG. 1b is shown removal of electrodes after alignment and thusfreestanding aligned film even in the case where the matrix is adhesive.The alignment also occurs if the electrodes do not touch the materialand so the alignment can be performed from the distance. When thematerial and electrodes are moved, continuous or stepwise, with respectto each other during the alignment, this allows continuous alignmentprocessing and different geometries. Three possible options for thealignment settings are illustrated in FIG. 1b that shows aligned filmwith (A-B) and without (C-D) electrical contacts between electrodes (a)and material (b). In the case (A) the aligned film forms permanentconnection between the electrodes. In the case (B) the electrodes andmaterial are only loosely joined together and can be moved apart afteralignment. In the case (C) there are insulating layers (c) between thematerial and electrodes and they are easily moved apart after thealignment even in the case where the material is an adhesive. In thiscase the obtained material is a multilayer consisting of aligned layer(b) and two insulating layers (c) In the case (D) the alignment iscarried out from the distance and the mutual location of electrodes andfilm can be additionally moved during the alignment. For illustrativepurposes the placement of the electrodes are shown such that alignmentoccurs in the z-direction. Alignment in the x- and y-direction or anarbitrary direction can be achieved by relative movement of the field,such as moving the distant electrodes.

FIG. 2 shows a picture of a single pathway consisting of an aligned rowof particles.

Example 3

This example concerns the applicability of the alignment method, the useof alignment for formation of individual aligned chains in thepredetermined positions.

The procedure was otherwise similar to that in Example 1 but instead ofgeneric surface the particle chain is aligned on the AFM cantilever.When the cantilever is bending, influenced due to the changing surfaceforces, the aligned pathway gets microscopically stretched and theparticles become disconnected from each other. This influences theconductivity through the particle chain. This allows the use of alignedchain as a sensor of surface properties of the surface studied by thecantilever. This setting is illustrated in FIG. 3, where the upper imageto the right illustrates a connected path, and the lower image (bentstate) illustrates a disconnected path.

Example 4

A resistive touchscreen placed in front of the display is created fromtwo layers of film with conductive pathways in x and y direction, andwith the matrix reduced so that the pathways are exposed. When contactis made to the surface of the touchscreen, the two sheets are pressedtogether. The horizontal and vertical pathways that when pushedtogether, let the controller register the precise location of the touchfrom any object, e.g. finger, stylus, pen, hand, by forming a contact.

In an alternative embodiment of this example the pathways in the x and ydirections are formed as two layers in a single film. During operationof a four-wire touchscreen, a uniform, unidirectional voltage gradientis applied to the first layer, using the two wires to electrodes at eachend of the sheet. The horizontal and vertical lines that are in thedeformed area will be broken because the carbon particles in theconductive pathways will separated. When the sheet is pressed, thecontroller measures the voltage as distance along the first sheet,providing the X coordinate. When this coordinate has been acquired, theuniform voltage gradient is applied to the second layer using the twoother wires to ascertain the Y coordinate. These operations occur withina few milliseconds, registering the touch location as contact is made.

Such a touchscreens typically have high resolution (4096×4096 DPI orhigher), providing accurate touch control.

Due to the low particle loading the touchscreens will be moretransparent, as the pathways will be practically invisible.

Example 5

A capacitive touchscreen is created from one layer of film withconductive pathways aligned in any direction. The pathways forms acapacitor that holds charge, e.g. from a voltage applied to the edges ofthe layer creating a controlled capacitor. When contact is made to thesurface of the touchscreen, or a finger of a dielectric or conductivebody is close to the touchscreen, the electric field across thetouchscreen is changed

The capacitive controller connected to the screen calculates the X and Ycoordinates from the change in the capacitance as measured from the fourcorners of the film.

In another embodiment two or more layers are used, with eight or morewires to the corners of the film, four and four connecting to eachlayer, thus creating higher resolution when the controller switchesbetween reading the layers, or if multiple controllers are used.

Example 6

A hybrid screen is manufactured with resistive and capacitive layers asshowed in FIG. 5. There could be more than one resistive and capacitivelayer. In this embodiment both the position of proximity to and pressureon the sensor from e.g. a finger or stylus will be detected by thecontrollers and sent to the processor of the PC, mobile phone or similardevice. In FIG. 5, the reference numbers indicates the following:

-   4:1—Display-   4:2—Transparent capacitive sensor-   4:3—Transparent resistive sensor-   4:4—Capacitive controller-   4:5—Resistive controller-   4:6—Processor

Example 7

Touch sensors to be used as in examples 5, 6 and 7 are formed usingglass instead of a polymer as matrix.

Example 8

A strain gauge is formed by using a matrix that is an elastic polymer.As shown in FIGS. 6A-6C the conductive pathways have been formed to apattern suitable for a strain gauge by moving the field relative to thematrix. In a simpler embodiment the pattern is a straight line, or anyother suitable pattern.

In FIG. 6A, the pathways form a strain sensitive pattern (S) between twoterminals (T). When the matrix is tensioned (FIG. 6B), the area of thepathways narrows and the resistance increases, resulting in higherresistance between the terminals (T). When the matrix is compressed(FIG. 6C), the area thickens and the resistance decreases, resulting ina lower resistance between terminals (T).

Example 9

An electronic hygrometer, a humidity sensor is manufactured usingcapacitive sensors similar to that in example 5, but where the change incapacity is due to a change in the amount of water present in thematrix. In another embodiment the change in conductivity is measured. Inone embodiment the matrix is made from cellulose. Temperature must alsobe measured, as it affects the calibration of these humidity sensors.

In one embodiment the alignment is in the z-plane, perpendicular to thesubstrate, and thus forming a structure similar to a carbon nanotubearray. The alignment can also be in the x, y plane. The absorption anddesorption of chemical vapours by the polymer matrix cause changes inthe inter-tube distance or the electrical properties of the matrix andthe conductance or capacitance changes.

Example 10

In order to produce a matrix with increased capacitance a proceduresimilar to that in example 1 was used, but the alignment was terminatedbefore the chains reached from electrode to electrode. FIG. 7 shows soobtained electrodes with dendritic surface. This creates pathways thatcan hold more charge than the conductive pathways else produced. Thecapacitance of this structure will be sensitive to deformation along thedirection indicated by an arrow in FIG. 7.

Example 11

The example with the procedures similar to any of the claim 1, 2, 3, 4,or 5 but instead of other previously employed particles, spiky particlesused in the QTCs are used.

Example 12

In this example aligned particles are used in a nanomechanicalcantilever. This means that the cantilever is highly miniaturized andthat instead of bulk layer the properties of single particles dominate.

Example 13

In this example a flexible sensor shaped as a thin sheet, coating orfilm is formed. It could be formed as part of the structure or as amaterial that is added on the whole or a part of the inner or outersurface of a body of a ship's hull, an aircraft or another vehicle orpart there of (such as a car's engine), industrial machinery or onbuildings, such as bridges or houses. It could also be used forpackaging or as part of clothing, furniture or electronic equipment suchas computers.

Example 14

This example is similar to example 13 but in addition the sensorfunction as at least one of an anti-static coating, a thermal conductor,an antenna and a shielding for electro-magnetic waves due to theproperties of the aligned pathways and the matrix that makes up thesensor.

Example 15

A micro-mechanical strain sensor made of carbon black (CB) particlesassembled into a single wire in a polymer matrix was produced.

The experimental procedure is shown in FIG. 8A-8D. CB particles (AlfaAesar) were dispersed in UV-curable urethane methacrylate-basedthermoset polymer Dymax 3094 (Dymax Corporation, CT). The particlefraction was 0.1 vol. %. This dispersion was spread to form a<10 μmlayer on top of tiplike gold electrodes (FIG. 8A).

These electrodes were prepared using UV-lithography on a 250 μm thicksilicon substrate covered by a insulating silicon oxide layer. Theirthickness, width and mutual spacing were 100 nm, 3 μm and 100 μm,respectively. In the next step an alternating electric field ofamplitude 3 kV/cm with a frequency of 1 kHz was applied over the sampleusing a custom-made voltage source. This led to the assembly of aparticle string in between the electrode tips, FIG. 8B, in less than twominutes. The material was subsequently UV-cured by a mercury lamp. Theresistance over the electrodes was monitored using a Keithley 2000multimeter.

FIGS. 8A-8D illustrate the experimental procedure. In FIG. 8A, a lowparticle fraction mixture was spread between the electrodes and alignedby an E-field into single strings by alternating electric field betweenthe electrode tips in FIG. 8B. In FIG. 8C it is illustrated how thissubstrate was bent and the resistance of the strings was measured as afunction of deflection. In FIG. 8D, W(x) is the vertical deflection andL is the length of the beam clamped at both ends.

The electromechanical properties of so prepared strings were studied byclamping the substrates under two clamps and bending them, as seen inFIG. 8C and FIG. 8D.

The substrates were bent at the substrate centre, which leads to thestretching of the string on the surface. The resistivity of the stringwas measured as a function of the vertical deflection W(x) FIG. 8D. Thesurface strain and the resistivity increase with increasing bending. Thestrain on the surface corresponds to the strain in the string and theresistance through the string is increased with this strain.

FIG. 9A and FIG. 9B show the impedivity and phase angle θ, respectivelyas function of frequency of the aligned and cured CB particle string.The impedivity is nearly constant up to 1 kHz and decreases for higherfrequencies. The phase angle begins to deviate from zero at 100 Hz,indicating a contribution from the capacitive conductivity.

FIG. 9A illustrates impedivity and FIG. 9B illustrates phase angle of analigned CB string in cured polymer.

FIG. 10A shows the resistivity of the single string in a cured polymermatrix as a function of deflection. Also shown are corresponding datafor the nonaligned film containing 12 vol. % of CB particles, i.e., afraction well above the percolation threshold. The strain correspondingto a given deflection is also presented in the graph and is the relativedisplacement of particles in the polymer with deformation. The verticaldotted lines mark the change of deflection direction. The samples werebent from a relaxed state at 0 μm to a deflected state at 50 μm, thenback to the relaxed state, and lastly bent to 50 μm once more. Thesemeasurements were done subsequently with the same samples. For thealigned sample, the data show an increase in resistivity with 50 μmdeflection. The original resistivity is restored on the release and theincrease is again seen with next deflection.

FIG. 10A illustrates the resistivity p of an aligned string of CBparticles (circles) with the initial 0.1 vol. % particle loading, and anon-aligned sample with the 12 vol. % particle loading, (squares) asfunction of deflection D. The dotted lines mark the change of deflectiondirection. FIG. 10B illustrates the relative change in resistance forthe aligned string as a function of deflection. The open triangles anddiamonds show the first and third deflection, respectively. The solidsquares show the first release. Dashed, solid and dash dotted lines arecorresponding linear fits.

FIG. 10B shows the relative change in resistance for the first andsecond deflection plus the first release of the aligned samplecorresponding to FIG. 10A. A gauge factor estimated for the alignedstring is about 150 with an error margin of 10%, as estimated from theseslopes using Eq. 1 (ΔR/R=K·S, where K is the gauge factor and S is thestrain). This is significantly higher than that of a nonaligned,high-particle fraction sample, which did not show any measurable effectdue to the stretching. This means that the alignment has significantbenefits in both the conductivity enhancement and in the strainsensitivity.

Results reported earlier (by Gammelgaard et al.) show that a SU8 polymermixed with high concentration (16%) of isotropic CBs have a gauge factorof 15-20. Thus a gauge factor about 10 times higher may be obtained withan aligned single string compared to an isotopic high particle fractionsample.

In conclusion, single strings of CB particles were aligned bydielectrophoresis in UV-curable Dymax 3094 polymer matrix and shown tobe a promising candidate as a strain sensor. By deforming these singlestrings in-plane, a reversible change in resistivity was observed,similar to what has been reported before 8 for non-aligned CB-SU8polymer composite when the particle fraction exceeds the percolationthreshold. A gauge factor of 150 was found, exceeding the value of 15-20reported previously. Detection of significant gauge factors fornonaligned CB-Dymax 3094 composites with the particle fraction exceedingthe threshold was not possible.

Higher gauge factors could be achieved by using different particlesizes, particles size distributions or by improving the conductivity ofthe nanoparticles.

Example 16

In another embodiment, carbon black particles were aligned in anelastomeric matrix. The particles were carbon black particles 0.0004 g,and the elastomer Dow Corning 734 Flowable Sealant (silicone basedelastomer), 0.6700 g. A solvent, 2-Butanone, 0.5941 g were used todecrease the viscosity of the elastomer. The particle concentration was0.03 wt %.

The resulting elastomeric matrix including aligned CB particlesdisplayed behaviour where the resistance of the conductive path of CBparticles decreased with compression of the elastomer.

FIG. 11 illustrates the resistance through the conductive path versuscompression of out-of-plane aligned CB articles in the matrix of DowCorning 734. This means that the sample is sheetlike and the alignedstrings are formed parallel to its surface normal, thus connecting twolargest surfaces through the sheet. The electrode had an initial spacingof about 150 μm. The data correspond to three subsequent compressions.It is seen how the resistance decreases with each compression andresumes a higher value when the compression is released. An alternatingelectric field of 1.5 kV/cm for 5 min. The sample was humidity-cured for24 hours. The resistances were measured with Keithley 2000 millimeter.

Example 17

Alignment of single strings of CNCs in Dymax 3094 polymer was performed.CNCs were mixed with urethane methacrylate based Dymax 3094 with aparticle fraction of 0.1 vol. %. This particle fraction is an order ofmagnitude lower than expected percolation threshold (˜2 vol. %). The lowparticle fraction suppresses aggregation thereby rendering a uniformmixture with the particle size below 3. Since the size of CNC particlesare between 100 nm and 3 the CNCs are believed to be nearly perfectlydispersed. The alignment was done by spreading a thin (1-10 μm) layer ofthis dispersion over the tip-like electrodes and applying an alternatingelectric field between the electrodes followed by UV-curing, as seen inFIGS. 8A-8C (details can be found in the Experimental section below).

FIG. 12 Multiple aligned strings of CNCs in a cured Dymax 3094 polymeron interdigidated electrodes.

FIGS. 13A to 13D show micrographs of the assembly of a string of CNCsover time. The applied field is E=4 kV/cm over an electrode spacing of100 μm. Originally isotropic mixture had the particle fraction of about0.1 vol. %. FIG. 13A illustrates particles dispersed in the polymerbefore the electric field was applied. Snapshots of the alignmentprocess after 45 seconds FIG. 13B and 1 minute 20 seconds FIG. 13C. Acomplete string was formed within 2 minutes 20 seconds FIG. 13D.

Hence, a conducting string between two electrodes with a spacing of 100μm was produced in less than 150 seconds.

Prior to the alignment, the resistance of the mixture is in the MΩrange. The alignment causes the resistance to drop over three orders ofmagnitude to the kΩ range. For instance, the aligned string shown inFIG. 13D had a resistivity of p=40 mΩ·m. UV-curing of the polymercomposite will locks the aligned carbon particles, making electricalcharacterization of the aligned strings possible. The alignment andconductivity are maintained upon curing.

FIG. 14 shows a close-up of an aligned string after curing with aresistivity of p=32 mΩ·m. This is an UV-cured single string of alignedCNC particles spanning an electrode gap of 100 μm.

Experimental

Materials and Sample Preparation

The samples contained carbon particles dispersed in a polymer matrix.The employed CNC material was supplied by n-TEC AS (Norway) and itcontained about 70% discs, 20% nanocones and 10% carbon black. Thematerial had been heat treated to 2700° C. prior use. The employed CBwas supplied by Alfa Aesar. The polymer used was Dymax 3094 UltraLight-Weld (Dymax Corporation, CT) supplied by Lindberg & Lund AS(Norway). This is an urethane methacrylate based UV-curable thermosetpolymer. Carbon particles were dispersed in the polymer by stirring at150 rpm for 15 minutes, which leads to a uniform dispersion with theparticle size less than 10 μm.

The alignment procedure is shown in FIGS. 8A-8C. The gold electrodeswere made by UV-lithography on a 250 μm thick silicon wafer covered by a300 nm thick insulating silicon oxide layer, and consisted of two 100 nmthick and 3 μm wide fingers facing each other with the spacing d rangingfrom 10 to 100 μm.

A layer (<10 μm) of dispersion was smeared on top of the electrodes(FIG. 8A). An electric field of ˜4 kV/cm with a frequency of 1 kHz wasapplied over the electrodes (FIG. 8B). The dielectrophoresis effectcauses the particles to move towards the two electrode tips, formingcontinuous strings at the edge of the tips. The strings would grow fromeach electrode tip until they met at the halfway point of the electrodegap forming a continuous conducting string (FIG. 8C). The alignmentoccurs within 1-3 minutes, depending on particle concentration and theapplied electric field. The particles will stay in place after theelectric field is turned off, but the characterization or moving of thesample may destroy the aligned string. The polymer is thereforesubsequently UV-cured by a mercury lamp for 5 minutes, locking theparticles into place.

For the DC conductivity measurements, 20 similarly prepared parallelsamples of CNCs and CB particles were aligned in Dymax 3094. Theresistance was monitored by a Keithley 2000 multimeter over thealignment electrodes immediately after alignment without UV-curing.

Electrical and Electromechanical Characterization

The electromechanical experiment is illustrated in FIG. 8D. The sampleswere clamped at both ends and a micrometer screw was used to control asmall blade at the center of the substrate. The blade would begin tobend sample thus stretching the carbon string on its upper surface. Thesamples were deflect by a given deflection and the relaxed, and thenbent yet again several times in a continuous manner. An IV curve wasmeasured for every deflection point and used for determining theresistance for each point. The resistance through the sample increaseswith increasing bending.

Electrical Properties of Aligned Strings

The DC conductivity of 20 identically aligned samples prepared inaccordance with the above were measured for both CNCs and CB particles.The distributions of these conductivities are shown in FIG. 15. FIG. 15illustrates the sample fraction versus conductivity for aligned CB(striped) and CNC (black) in Dymax 3094 polymer. The CNCs have higherconductivity but have a smaller probability to produce a conductingstring. Only fifty percent of the aligned single strings of CNC wereconducting and the nonconducting strings are not included in the shownprobability distribution.

The figure is normalized so that the probability of producing aconductive string is unity in both cases. The CNC strings have a higherconductivity than the CB strings.

The CNC particle strings have conductivities ranging from 25 to 500 S/m,with 90% falling in the region below 100 S/m. The CB particles haveconductivities ranging from 1 to 22 S/m, with 95% falling in the regionbelow 6 S/m. Though the conductive CNC particle strings have a higherconductivity, only 10 out of 20 prepared samples conducted anymeasureable current. These nonconductive strings are not included inFIG. 15. However, all 20 the CNC particle strings appeared visuallycomplete and were therefore expected to be conductive. This implies thattiny, optically invisible mismatches between particles are enough toprevent a conductive pathway. One reason for this difference betweenCNCs and CB may stem from the different particle topology. Anotherreason may be the polydisperse nature of the CNC particles. The conesand discs might have difficulties creating good “topologically matching”connection between each other due to the variable shape of theparticles.

The conductivity of an intact string provides an estimation for theuppermost conductivity of the earlier reported multi-string samples likethe one shown in FIG. 12. As the former is significantly higher (forCNCs 25-500 S/m, see FIG. 15) than the conductivity of multi-stringsamples normalized to the volume fraction of particles (0.1-1 S/m), thisindicates that a large part of seemingly intact strings in themulti-string samples (FIG. 12) are actually broken. The conductivity gapbetween single strings and multi-string samples could potentially bereduced by optimizing the preparation of larger samples for exampleusing even and vibration free preparation setups.

FIG. 16 plots the current-voltage curves of an aligned low particlefraction sample and a nonaligned high particle fraction sample. Thesolid line is from an aligned sample prepared from the isotropicCNC-polymer mixture with a concentration of 0.1 vol. %, while the dashedline represents the nonaligned sample with a concentration of 13 vol. %.The direct current through both samples grow linearly with increasingvoltage up to 100 mV and no sign of hysteresis is observed on cycling.The nonaligned sample has a marginally higher conductivity than thealigned one but they are both of the same order of magnitude.

FIG. 17 The direct current through the sample grows linearly withincreasing voltage up to 100 mV and no sign of hysteresis is observed oncycling.

FIGS. 18A and 18B plot the impedivity and phase angle vs. frequency forboth an aligned sample and a non-aligned, high particle-fraction sample.AC impedance data of aligned CNC strings (squares) and isotropic CNCfilm with high particle fraction (13 vol %) in Dymax 3094 polymer(triangles). FIG. 18A impeditivity as a function of frequency, FIG. 18Bphase angle as a function of frequency.

Both samples behave very similarly when the frequency is increased. Theimpedivity is nearly constant up to 1 kHz, but falls noteworthy between1 kHz and 10 kHz. The phase angle begins to deviate from zero at 100 Hz,indicating the rise of capacitive conductivity. The similarity betweenaligned and nonaligned samples implies that the aligned string behavesessentially as bulk, heavily loaded CNC composite. These data are alsoconsistent with the AC-impedance of multi-string samples indicating theOhmic nature of the strings. However, the data differs from the data ofnonaligned CNC polymer mixtures at low particle fraction where the phaseangle begins to differ from zero much earlier (>10 Hz), pointing to theionic conductivity of the polymer.

Electromechanical Properties of Aligned Strings

Next, electromechanical measurements of the CNC strings in cured matrixwere performed. The samples were clamped at both ends and deflectedgradually in the centre as shown in FIG. 8D. The deflection leads to thestretching of the surface layer and thereby causes an increasing strainof the aligned film and presumably moves the particles with respect toeach other. The measurements were conducted by deflecting the samplesseveral times and measuring the resistivity for every deflection point.

FIG. 19A illustrates resistivity as a function of deflection for analigned CNC string (open squares) and nonaligned sample with high 13vol. % particle fraction (solid square). The dotted lines mark thechange in deflection direction. FIG. 19B The relative change inresistance as a function of deflection for the aligned string. The opentriangles show the first deflection while the open diamonds representthe third deflection. The solid squares show the first release. Dashed,dash dotted and solid lines are corresponding linear fits.

FIG. 19A shows the so obtained resistivities for an aligned sample as afunction of deflection. The corresponding data of nonaligned sample isshown for comparison. The aligned sample was prepared from thedispersion with a particle fraction of 0.1 vol. % and is the exact samesample as shown in FIG. 14. The nonaligned sample had a particleconcentration of 13 vol. %. The resistivity at each deflection point wascalculated by linear fits to separate IV curves measured at each point.This method is justified by the ohmic behaviour of strings for DC andlow measurement frequencies. The top axis shows the induced strain ofthe strings defined as the relative displacement of particles due to anapplied external force. The measurements were done in a continuousmanner and the vertically dotted lines mark the change of deflectiondirection. When the deflection goes from 0 to 50 μm to 30 μm in FIG.19A, it means that the sample was deflected from a relaxed state at 0deflection to a deflected state at 50 μm and relaxed to a less deflectedstate at 30 μm. The deflection has a significant and reversible effecton the resistivity of the aligned sample, the resistivity increasingwith the increasing deflection and strain. In contrast, the resistivityof nonaligned sample does not show any measureable effect.

FIG. 19B shows the relative change in resistance calculated from thefirst and second deflections as well as the first release of the alignedsample (FIG. 19A). These data gives a gauge factor of 50 with an errormargin of 10%, as estimated from the slopes in FIG. 19B, using Eq. 1.This shows that the alignment has a significant effect making bothconductive and piezoresistive CNC materials possible. The aligned stringperforms well against its standards of comparison. The obtained gaugefactor is higher than those of typical thick-film resistors, whose gaugefactors range from 3-30 and also compares well to that of an optimizedsilicon piezoresistive cantilever. The gauge factor is also notablyhigher than typical values reported for aligned multi-walled carbonnanotubes, for which values of about 1.5 and 3 have been reported forparticle concentrations of 0.75 wt-% and 0.5 wt-%, respectively.

In this work, the strain was calculated at the top surface of thesubstrate, and it was assumed that the strain experienced by thecomposite layer with the particle string, was the same. This is onlycorrect if the composite layer thickness is well below the substratethickness, but this condition was fulfilled in our experiments.

It will be understood that a person skilled in the art may readilyenvisage numerous alternatives to the above-described exampleembodiments. In particular, the described features may be varied orcombined to form new embodiments.

The invention claimed is:
 1. A capacitive sensor comprising one or moreconductive pathways formed from conductive particles in a matrix,wherein the conductive particles comprise carbon particles or silverparticles, wherein the number of particles in the pathways is below thenumber of particles that constitutes a percolation threshold if theparticles were homogenously distributed in the matrix.
 2. The capacitivesensor of claim 1, wherein the capacitive sensor comprises a substrate,and the conductivity of the aligned particles is influenced by thesubstrate deformations or bending.
 3. A hybrid sensor, comprising: thecapacitive sensor of claim 1 and a resistive sensor, wherein thecapacitance of the pathways is influenced by the presence of an objector a human.
 4. The capacitive sensor of claim 1, wherein the conductiveparticles have an aspect ratio range of 1-20.
 5. The capacitive sensorof claim 1, wherein the conductive particles comprise carbon particlesselected from the group consisting of irregular graphitic particles,spherical carbon black (CB) particles, disc-like particles, and conicalcarbon particles (carbon nanocones CNCs).
 6. The capacitive sensor ofclaim 1, wherein the matrix is a thermoset polymer, a thermoplasticpolymer system, a lyotropic system or a mixture thereof.
 7. Thecapacitive sensor of claim 1, wherein the matrix comprises a UV-curablepolymer.
 8. The capacitive sensor of claim 1, wherein the matrixcomprises an elastomer.
 9. The capacitive sensor of claim 1, wherein theconductive particles comprise CB particles.
 10. The capacitive sensor ofclaim 1, wherein the matrix comprises a polymer material.
 11. Thecapacitive sensor of claim 1, wherein said capacitive sensor is amicro-mechanical strain sensor.
 12. A cantilever sensor, comprising thecapacitive sensor in accordance with claim
 1. 13. A nanomechanicalcantilever sensor, comprising the capacitive sensor in accordance withclaim
 1. 14. A vapour sensor comprising the capacitive sensor inaccordance with claim
 1. 15. The capacitive sensor of claim 1, where thecapacitive sensor also functions as at least one of an anti-staticcoating, a thermal conductor, an antenna and an electro-magneticshielding.
 16. A robot skin application, comprising the capacitivesensor in accordance with claim
 1. 17. A moisture sensor comprising thecapacitive sensor in accordance with claim
 1. 18. A method fornonintrusive bio monitoring, the method comprising: measuring a pulse ofa patient in need thereof as strain on the patient's skin with thecapacitive sensor of claim
 1. 19. The method of claim 18, wherein thecapacitive sensor measures a change in capacitance due to the patient'sheartbeat.
 20. A method for nonintrusive bio monitoring, the methodcomprising: measuring moisture or vapour from a patient in need thereofas changes in conductance or capacitance with the capacitive sensor ofclaim 1.